Important properties desirable in tyre treads include good adhesion on dry and wet surfaces, low rolling resistance and high abrasion resistance. It is very difficult to improve the skid resistance of a tyre without simultaneously worsening the rolling resistance and abrasion resistance. A low rolling resistance is important for low fuel consumption, and high abrasion resistance is a crucial factor for a long service life of the tyre.
Wet skid resistance and rolling resistance of a tyre tread depend largely on the dynamic/mechanical properties of the rubbers which are used in the blend production. To lower the rolling resistance, rubbers with a high resilience at higher temperatures (60° C. to 100° C.) are used for the tyre tread. On the other hand, for improving the wet skid resistance, rubbers having a high damping factor at low temperatures (0 to 23° C.) or low resilience in the range of 0° C. to 23° C. are advantageous. In order to fulfil this complex profile of requirements, mixtures of various rubbers are used in the tread. Usually, mixtures of one or more rubbers having a relatively high glass transition temperature, such as styrene-butadiene rubber, and one or more rubbers having a relatively low glass transition temperature, such as polybutadiene having a high 1,4-cis content or a styrene-butadiene rubber having a low styrene and low vinyl content or a polybutadiene prepared in solution and having a moderate 1,4-cis and low vinyl content, are used.
Anionically polymerized solution rubbers containing double bonds, such as solution polybutadiene and solution styrene-butadiene rubbers, have advantages over corresponding emulsion rubbers in terms of production of tyre treads with low rolling resistance. The advantages lie, inter alia, in the controllability of the vinyl content and of the associated glass transition temperature and molecular branching. In practical use, these give rise to particular advantages in the relationship between wet skid resistance and rolling resistance of the tyre.
Important contributions to energy dissipation and hence to rolling resistance in tyre treads result from free ends of the polymer chains and from the reversible buildup and degradation of the filler network formed by the filler used in the tyre tread mixture (usually silica and/or carbon black).
The introduction of functional groups at the end of the polymer chains and/or start of the polymer chains enables physical or chemical attachment of these ends and/or starts of the chains to the filler surface. This restricts the mobility thereof and hence reduces energy dissipation under dynamic stress on the tyre tread. At the same time, these functional groups can improve the dispersion of the filler in the tyre tread, which can lead to a weakening of the filler network and hence to further lowering of the rolling resistance.
For this purpose, numerous methods for end group modification have been developed. For example, EP0180141A1 describes the use of 4,4′-bis(dimethylamino)benzophenone or N-methylcaprolactam as functionalizing reagents. The use of ethylene oxide and N-vinylpyrrolidone is also known from EP0864606A1. A number of further possible functionalizing reagents are detailed in U.S. Pat. No. 4,417,029. Methods for introducing functional groups at the start of the polymer chains by means of functional anionic polymerization initiators are described, for example, in EP0513217A1 and EP0675140A1 (initiators with a protected hydroxyl group), US20080308204A1 (thioether-containing initiators) and in U.S. Pat. No. 5,792,820, EP0590490A1 and EP0594107A1 (alkali metal amides of secondary amines as polymerization initiators).
The carboxyl group, as a strongly polar, bidentate ligand, can interact particularly well with the surface of the silica filler in the rubber mixture. Methods for introducing carboxyl groups along the polymer chain of diene rubbers prepared in solution are known and are described, for example, in DE2653144A1, EP1000971A1, EP1050545A1, WO2009034001A1. These methods have several disadvantages, for example that long reaction times are required, that the functionalizing reagents are converted only incompletely, and that an alteration of the polymer chains occurs as a result of side reactions such as branching. Moreover, these methods do not enable particularly effective functionalization of the ends of the polymer chain.
The introduction of carboxyl groups at the chain ends of diene rubbers has likewise been described, for example in U.S. Pat. No. 3,242,129, by reaction of the anionic ends of the polymer chain with CO2. This method has the disadvantage that the polymer solution has to be contacted with gaseous CO2, which is found to be difficult because of the high viscosity and the resultant poor mixing. In addition, coupling reactions which are difficult to control occur as a result of reaction of more than one end of the polymer chain at the carbon atom of the CO2. This coupling can be avoided by sequential reaction of the carbanionic ends of the polymer chain first with ethylene oxide or propylene oxide, followed by reaction of the ends of the polymer chain which are now alkoxidic with a cyclic anhydride (U.S. Pat. No. 4,465,809). Here too, however, there is the disadvantage that gaseous and additionally very toxic ethylene oxide or propylene oxide has to be introduced into the high-viscosity rubber solution. Furthermore, reaction of the alkoxidic chain ends with the cyclic anhydride forms hydrolysis-prone ester bonds which can be cleaved in the course of workup and in the course of later use.
Especially silanes and cyclosiloxanes having a total of at least two halogen and/or alkoxy and/or aryloxy substituents on silicon are of good suitability for end group functionalization of diene rubbers, since one of said substituents on the silicon atom can be readily exchanged in a rapid substitution reaction for an anionic diene end of the polymer chain and the further aforementioned substituent(s) on Si is/are available as a functional group which, optionally after hydrolysis, can interact with the filler of the tyre tread mixture. Examples of silanes of this kind can be found in U.S. Pat. No. 3,244,664, U.S. Pat. No. 4,185,042, EP0778311A1 and US20050203251A1.
These silanes generally have functional groups which are bonded directly to the silicon atom or bonded to Si via a spacer and can interact with the surface of the silica filler in the rubber mixture. These functional groups are generally alkoxy groups or halogens directly on Si, and tertiary amino substituents bonded to Si via a spacer. Disadvantages of these silanes are the possible reaction of a plurality of anionic ends of the polymer chain per silane molecule, elimination of troublesome components and coupling to form Si—O—Si bonds in the course of workup and storage. The introduction of carboxyl groups by means of these silanes has not been described.
WO2012/065908A1 describes 1-oxa-2-silacycloalkanes as functionalizing reagents for introduction of hydroxyl end groups in diene polymers. These 1-oxa-2-silacycloalkanes do not have the disadvantages of the silanes described in the above paragraph, such as reaction of a plurality of anionic ends of the polymer chain per silane molecule, elimination of troublesome components and coupling to form Si—O—Si bonds in the course of workup and storage. However, these functionalizing reagents also do not enable the introduction of carboxyl groups at the ends of the polymer chain.
The problem addressed was therefore that of providing carboxyl-terminated polymers which do not have the disadvantages of the prior art and more particularly enable utilization of the good reactivity of silanes having anionic ends of the polymer chain.